In recent years, the prevailing are image forming devices, such as copiers and printers, that enable double sided printing by forming an image on a front side of a sheet, and then reversing the front and back sides of the sheet. to form an image on the back side. Users have various demands on printings for which such image forming devices performs double sided printing. FIG. 8 shows diagrams of a sheet S having been subjected to double-sided printing, wherein the diagram on the left side in FIG. 8 shows the image on the front side, and the diagram on the right side shows the image on the back side. The area inside the dashed lines in FIG. 8 is the image area, and the position of the image on the front side with respect to the image on the back side is shown G in FIG. 8. For a double-sided printing sheet S, as shown in FIG. 8, there is a demand that an image on the front side and an image on the back side be formed at the same position (X in FIG. 8) relative to the same edge of the sheet. Particularly, printing industries, where strict conditions regarding printings are imposed, have a strong requirement.
In this situation, there are proposed technologies of superposing the positions of an image on the front side and an image on the back side of a sheet to be subjected to double-sided printing. According to the technology disclosed in Patent Document 1 (Japanese Patent Publication TOKKAI No. H11-237768), when an image is to be recorded on the front side of a recording medium, the relation between the trailing edge position of a recording sheet and the position of image recording is detected, the position being in the conveying direction of the recording medium; when an image is to be recorded on the back side of the recording medium, the leading edge position of the recording medium in the conveying direction is detected; and an image recording position of the back side of the recording sheet is set, based on a thus detected signal. Further, according to the technology disclosed in Patent Document 2 (Japanese Patent Publication TOKKAI No. 2000-305324), dedicated positioning marks are formed in a margin of a sheet material having been subjected to a fixing process; the marks are read with a CCD line sensor in case of forming an image on the back side; and position adjustment of the image on the back side is performed.
However, with the technology disclosed in Patent Document 1, thermal shrinkage is caused when a recording sheet having an image recorded on the front side passes through the fixing device. Accordingly, even when an image is recorded on the back side with reference to the position of the leading edge of the recording sheet in the conveying direction, it may occur that the image on the front side and the image on the back side are formed at different positions from the same edge.
With the technology disclosed in Patent Document 2, dedicated positioning marks are formed on a sheet. Accordingly, even if marks are formed of yellow toner, which is obscure, an image which is not requested by the user is formed, which is not preferable.
Therefore, in accordance with the present invention, there is provides an image forming apparatus that superposes an image on the front side and an image on the back side by a suitable method, based on setting information on double-sided printing.
To achieve at least one of the abovementioned objects, an image forming apparatus reflecting one aspect of the present invention forms an image on a front side of a sheet, then reverses the sheet front and back sides, and forms an image on the back side of the sheet, and comprises:
a conveying path for conveying the sheet;
an edge portion detection sensor which detects an edge of the sheet being conveyed through the conveying path, the edge portion detection sensor provided on the conveying path;
a mark detection sensor which detects a reference mark formed on the sheet being conveyed through the conveying path, the mark detection sensor provided on the conveying path;
a first determination section which determines a position of an image to be formed on the back side of the sheet, referring to the edge of the sheet detected by the edge portion detection sensor;
a second determination section which determines a position of an image to be formed on the back side of the sheet, referring to the reference mark detected by the mark detection sensor; and
a switching section which switches whether to use the first determination section or the second determination section for determination of the position of the image to be formed on the back side of the sheet, based on setting information on double-sided printing to be performed by the image forming apparatus.